1. Field of the Invention
This invention relates to a process for wedge bonding gold wire to an aluminum bond pad at room temperature without the use of a heated work holder to enhance required bond adhesion.
2. Brief Description of the Prior Art
An integral procedure required in the fabrication of semiconductor devices is that of bonding of wires to bond pads and the like. Several factors must be addressed in the wire bonding procedure, among these factors being adequacy of adhesion of the bonded parts to each other, possible reaction between or among the materials involved, the time required to complete the bonding operation and the conditions under which bonding is to take place.
One highly successful bonding procedure is ultrasonic bonding. Ultrasonic bonding has been accomplished in the prior art at various frequencies up to about 125 KHz in the prior art. Such bonds have principally been made between gold bond wires and aluminum or aluminum alloy surfaces on integrated circuits. Interconnections have been made between aluminum wires and aluminum surfaces, however these interconnections have generally been made at a frequency of about 60 KHz. Such interconnections are described in a paper entitled "Effect of Ultrasonic Vibration on Wire Bonding" by Hiroshi Haji et al., Department of Materials Science and Technology, graduate School of Engineering Sciences, Kyushu University, Kasuga, Fukuoka, Japan. The aluminum to aluminum interconnections made at this lower frequency has not produced a sufficient metal interchange between the bond wire and the bond surface to produce a strong bond. Hence, a process is required to achieve, through metal interchange, a strong bond wire to bond surface connection at lower temperature.
Bonds have been made at 120 KHz as described in an article in the ICEMM Proceedings of 1993, "High Reliability Wire Bonding Technology by the 120 KHz Frequency of Ultrasonic", by Yuji Shirai et al. This article shows a reduced wire bond failure rate at 120 KHz and discusses gold wedge bonding at room temperature.
U.S. Pat. No. 5,660,319, which is assigned to the assignee of the subject application, provides an improvement over the prior art thereto by bonding an aluminum wire to an aluminum bond site utilizing an ultrasonic power source that is in the range of 150 KHz to 165 KHz and approximately 162 KHz. A mash force is applied from a pressure source during bonding. A metal exchange takes place between the aluminum wire and the aluminum bond site during the application of the ultrasonic power to produce a strong bond between the aluminum wire and the aluminum bond site.
A problem with prior art wedge bonding techniques is that such techniques have not provided ideal bonding conditions when used in conjunction with bonding of gold to aluminum. The bonding has taken place at elevated temperatures with present techniques requiring a heated work holder which is heated in the range of from about 120 to about 220 degrees C. to accommodate the needed interconnect. At such temperatures, the possibility of reactions taking place between or among the materials to be bonded is increased.